The present invention relates to a process for preparing an organic binder suitable for use in compacting an inorganic powder such as a powder of ceramic material (hereinafter referred to as "ceramic powder") or a metallic powder and sintering the resulting compact. More particularly, the present invention relates to a process for preparing an organic binder which has a good compactibility of inorganic powders in injection compacting or extrusion compacting and a good binder removability from green compact in the sintering step and which c,,,an provide sintered bodies having no or little defects and a good dimensional stability in high yields.
Sintered bodies have been prepared by compacting a mixture of an inorganic powder with an organic binder, and sintering the resulting compact. As the organic binder, there have been used, for instance, waxes and polymers such as ethylene-vinyl acetate copolymer (EVA), polystyrene, polypropylene, polyethylene and acrylic polymers. The organic binders used in the preparation of sintered bodies are required to have good characteristics such as compactibility (e.g. compacting stability and powder flowability in compacting inorganic powders to which the binders are added, the flowability of a mixture of an inorganic powder and a binder being hereinafter referred to as "flowability in compacting"), strength of green compacts, removability of binder from green compacts by thermal decomposition prior to the sintering, shape retainability in sintering and amount of residual carbon. However, known binders have both merits and demerits and are not always satisfactory in these characteristics.
For instance, the wax binders have a good removability, but have the drawbacks that they are poor in compactibility due to high crystallinity and the obtained green compacts are low in strength.
In general, the polymer binders are superior in compactibility, but have the drawback of being poor in removability. Mentioning the compactibility of polymer binders, polystyrene provides green compacts having a satisfactory strength and accordingly it is hard to cause jetting, but it is poor in powder flowability in compacting due to high melting point. Ethylene-vinyl acetate copolymer (EVA) is superior in powder flowability in compacting and provides an elasticity to green compacts, but defects such as blister and cracks are easy to occur in removing the binder by thermal decomposition. Acrylic polymers provide green compacts having a high strength, but are poor in mold releasing property of green compacts.
In view of such circumstances and in order to meet a recent demand for a binder which is suitable for the preparation of small-sized compacts having a complicated shape, it has been attempted to provide a mixed binder having well balanced characteristics by blending the acrylic polymer as a main component which is somewhat inferior in mold releasing property to other polymer binders, but is superior in binder removability, with other polymer binders such as polystyrene, ethylene-vinyl acetate copolymer or polyethylene. However, a difficulty is encountered in obtaining a homogeneous mixture by admixing such known polymer binders, because they differ from each other in form and softening point and also because the solubility or compatibility is not satisfactory. For example, the solubility parameters [SP values, (Cal/cm.sup.3).sup.1/2 of these polymers are 9.10 for polystyrene, 8.49 for a copolymer of ethylene and vinyl acetate in a ratio of 70:30 by weight, and 9.25 for polymethyl methacrylate. In a mixture of these polymers, the polymers are not compatible with each other and, therefore, if such a mixed polymer binder is admixed with an inorganic powder, no uniform mixture is obtained. A nonuniform mixture of an inorganic powder and a binder has problems that it takes time to determine the compacting conditions since the flowability in compacting is not stabilized, the yield is lowered, defects such as warpage, crack and sink mark are easy to occur in the obtained sintered bodies and the use of nonuniform mixture also has a bad effect on dimensional accuracy and density of the products.
It is an object of the present invention to provide an organic binder which is suitable for use in the preparation of a sintered body of an inorganic powder such as ceramic powder or metallic powder and which is superior in flowability of a mixture with the inorganic powder and in removability by thermal decomposition prior to sintering the green compact obtained from the mixture with the inorganic powder and which can provide in a high yield a sintered body having no defects such as warpage, crack and sink mark and having a high dimensional accuracy and a predetermined density.
Another object of the present invention is to provide a process for preparing an organic binder having excellent characteristics as mentioned above.
These and other objects of the present invention will become apparent from the description hereinafter.